Continuous web printing permits economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other substrate material is fed past one or more printing subsystems that form images by applying one or more colorants onto the substrate surface. In a conventional web-fed rotary press, for example, a web substrate is fed through one or more impression cylinders that perform contact printing, transferring ink from an imaging roller onto the web in a continuous manner.
Proper registration of the substrate to the printing device is of considerable importance in applications such as print reproduction, particularly where multiple colors are used in printing color images. Similarly, in the printing of electrical circuits, proper registration is critical in the deposition of electrically conductive or insulating layers in forming a multi-layer electrical circuit such as touch panels. Conventional web transport systems in today's commercial offset printers address the problem of web registration with high-precision alignment of machine elements. Typical of conventional web handling subsystems are heavy frame structures, precision-designed components, and complex and costly alignment procedures for precisely adjusting substrate transport between components and subsystems.
Alignment during actual print production is aided by vision systems monitoring the printed output in real time, comparing the output with a reference image and displaying the information to the operator to consider taking corrective actions. Such vision systems can monitor the color reproduction or the registration or both aspects of the print production to ensure the desired output quality.
The problem of maintaining precise and repeatable web registration and transport becomes even more acute with the development of high-resolution non-contact printing, such as high-volume inkjet printing. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from a print station onto the surface of the moving media, with the web substrate often coursing past the print station at speeds measured in hundreds of feet per minute. No impression roller is used; synchronization and timing are employed to determine the exact timing of the sequential deposition of ink by different print stations onto the moving media. The requirements for the printed output are driven by intended use and function of the printed product. For any multi-step printing process, the image quality attributes always include registration, print resolution and the reduction of print artifacts. Other attributes, specific to the output can be added, for example color reproduction for graphic arts printing. With dot resolution of 600 dots-per-inch (DPI) and better, a high degree of registration accuracy can be achieved theoretically, limited only by the digital resolution inherent in the digital print station. During printing, variable amounts of ink is applied to different portions of the rapidly moving web, with drying mechanisms typically employed after each print station or bank of print stations. Variability in ink or other liquid amounts and types and in drying time can cause substrate stiffness and tension characteristics to vary dynamically over a range for different types of substrate, contributing to the overall complexity of the substrate handling and registration challenge.
One approach to the registration problem is to provide a print module that forces the web of print media along a tightly controlled print path. This is the approach that is exemplified in U.S. Patent Publication No. 2009/0122126, entitled “Web Flow Path” by Ray et al. In such a system, there are multiple drive rollers that fix and constrain the web of print media position as it moves past one or more print stations.
Problems with such a conventional approach include significant cost in design, assembly, adjustment, and alignment of web handling components along the media path. While such a conventional approach permits some degree of modularity, it would be difficult and costly to expand or modify a system with this type of design. Each “module” for such a system would itself be a complete printing apparatus, or would require a complete, self-contained subassembly for paper transport, making it costly to modify or extend a printing operation, such as to add one or more additional colors or processing steps, for example.
Various approaches to web tracking are suitable for various printing technologies. For example, active alignment steering, as taught for an electrographic reproduction web (often referred to as a belt on which images are transported) in commonly assigned U.S. Pat. No. 4,572,417 entitled “Web Tracking Apparatus” to Joseph et al. would require multiple steering stations for continuous web printing, with accompanying synchronization control. It would be difficult and costly to employ such a solution with a print medium whose stiffness and tension vary during printing, as described above. Other solutions for web (or belt as referred to above) steering are similarly intended for endless webs in electro-photographic equipment but are not readily adaptable for use with paper media. Steering using a surface-contacting roller, useful for low-speed photographic printers and taught in commonly assigned U.S. Pat. No. 4,795,070 entitled “Web Tracking Apparatus” to Blanding et al. would be inappropriate for a surface that is variably wetted with ink and would also tend to introduce non-uniform tension in the cross-track direction. Other solutions taught for photographic media, such as those disclosed in commonly assigned U.S. Pat. No. 4,901,903 entitled “Web Guiding Apparatus” to Blanding are well suited to photographic media moving at slow to moderate speeds but are inappropriate for systems that need to accommodate a wide range of media, each with different characteristics, and transport each media type at speeds of hundreds of feet per minute.
In order for high-speed non-contact printers to compete against earlier types of devices in the commercial printing market, the high cost of the web transport should be greatly reduced. There is a need for an adaptable non-contact printing system that can be fabricated and configured without the cost of significant down-time, complex adjustment, and constraint on web of print media materials and types.
One aspect of such a system relates to components that feed the continuous web substrate into the printing system and guide the web of print media into a suitable cross-track position for subsequent transport and printing. This problem is exacerbated by the shrinking and expanding of web of print media due to wetting and drying. The change in the structure of the web of print media results in color-to-color registration errors during printing.
In other applications such as the manufacture of touch screens, the web of print media is typically made of plastic with a solvent based ink used in the printing process. Drying at elevated temperatures will change the dimensions of the support during the printing process much like in conventional printing applications.
In commercial inkjet printing systems, the web of print media is physically transported through the printing system at a high rate of speed. For example, the web of print media can travel 650 to 1000 feet per minute. The print stations in commercial inkjet printing systems typically include multiple jetting modules that jet ink onto the web of print media as the web of print media is being physically moved through the printing system. A reservoir containing ink or some other material is typically behind each nozzle plate in a print station. The ink streams through the nozzles in the nozzle plates when the reservoirs are pressurized.
The jetting modules in each print station in commercial printing systems typically jet only one color. In printing systems designed to manufacture electrical circuits, the jetting modules in each print station jet only electrically conductive inks, electrically insulating inks or inks to form protective coatings for the circuit. In printing systems designed for commercial printing or system designed to manufacture electrical circuits, the sequential deposition of inks along the conveyance path of the print media will form the printed product. The quality requirements and attributes of the printed product are derived from the use and application of the printed product. For example, in commercial printing systems the registration of the four colors forming the color image has to be performed precisely, the printed image should not have image artifacts and the overall color reproduction should resemble closely the color of the original object. In the manufacture of electrical circuits, the registration of the insulating and conductive layers should be performed precisely to avoid electrical short circuits. There should be no image artifacts such as voids affecting the electrical traces, making them non-conductive. Similarly, the crossing of two conductors not properly insulated from each other should be avoided. The current carrying capacity of each trace can require a certain density of conductive ink. For each of the example applications, the ink is jetted sequentially and deposited on the moving print media web as it is conveyed passed multiple print stations. In the examples, the printed output is composed of multiple layers, also referred to as separations, which should be aligned to each other to produce a single color impression for the observer of the commercial print or the desired function selected by the user on the touch screen panel forming the user interface.
The mis-alignment of layers or separations of a multi-layer print is typically referred to as registration error. Registration errors are partitioned into different types. Examples of registration errors include, but are not limited to, a separation having a linear translation with respect to another separation, a separation being rotated with respect to another separation, and a separation being stretched, contracted, or both stretched and contracted with respect to another separation. There are several variables that contribute to the registration errors in separation alignment including physical properties of the web of print media, conveyance of web of print media, ink application system, ink coverage, and drying of ink. Registration errors can be reduced by controlling these variables.
US 20140064817 discloses operating a printer at a fixed drive speed ratio during printing to reduce registration errors as compared to operating at a servo controlled tension. Stretch and tension are related through the elastic modulus of the web. If the modulus of the web is fluctuating due to inking of the paper and the tension is held constant then the stretch must vary to account for the changing modulus. On the other hand if a fixed speed ratio is maintained, yielding a fixed paper stretch, the tension must fluctuate to account for the modulus fluctuations. If the modulus of the paper fluctuates due to inking of the paper at least one of the stretch and the tension must fluctuate as well. If the tension is servoed so that it doesn't change, as in US 20140064817, then the stretch of the paper must fluctuate which hurts registration.
There is, then, a need for a tension control system that can reduce registration errors by controlling the conveyance of the web of print media in a high-speed commercial printing system for non-contact printing applications and compensate for varying tensions in the receiver web due to modulus changes of the material such as paper or plastic due to the sequential inking and drying steps employed to form the final image on the receiver web.